1. Field of the Invention
This invention relates generally to a system and method for operating a cooling fluid pump in a vehicle when the vehicle is not being used and, more particularly, to a system and method for operating a cooling fluid pump and heater that heats the cooling fluid pumped by the pump in a fuel cell or electrical hybrid vehicle when the vehicle is not being used.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is necessary that a fuel cell operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. The temperature provides the relative humidity for the fuel cells in the stack for a particular stack pressure. Excessive stack temperature above the optimum temperature may damage fuel cell components, reducing the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduces the stack performance.
Fuel cell systems employ thermal sub-systems that control the temperature within the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling channels in the bipolar plates in the stack. Typically the cooling fluid is a liquid that inhibits corrosion within the stack, does not freeze in cold environments, and is non-conductive. One example of a suitable cooling fluid is a de-ionized water and glycol mixture. It is necessary that the cooling fluid be non-conductive so that current does not travel across the cooling fluid channels in the stack.
At cold system start-up before the fuel cell stack has reached its desired operating temperature, the stack is generally unable to produce enough power to operate the vehicle. Therefore, the vehicle operator must wait a certain period of time until the fuel cell stack reaches its operating temperature as a result of stack inefficiencies before demanding significant load for operating the vehicle. For sub-zero system start-ups, the fuel cell stack may take a significant period of time to reach its operating temperature at which time it is able to provide power to operate the vehicle.
In sub-zero environments, water in the fuel cell stack and other system components, such as pipes and hoses, may freeze. It is known in the art to heat the cooling fluid and other structures in a fuel cell system using electrical heaters before and during cold system start-up to improve the system start-up time. It would be desirable to provide a system that prevented the fuel cell stack and related components from freezing during those time that the fuel cell vehicle is not being operated.